Well, after several months living with this frustrating bug, I was further frustrated to see that there were still no widely-publicized solutions. I decided to stop waiting and try to address the bug myself by simply removing Apple's trackpad driver from the equation.
To my (pleasant) surprise, installing the SideTrack 1.2 trackpad driver fixed the problem completely, driving idle kernel_task utilization down as low as 1%.
While I would rather not have to use SideTrack forever, not only has it improved my battery life and idle temperature, it also definitively demonstrates that, indeed, it is Apple's new 2-finger scrolling trackpad drivers in 10.4 which are to blame. Hopefully now they'll do something about fixing them in 10.4.3.

